Sync changes to v3 folder

Pull DIGEST-MD5 bind support changes into v3 (#230)

nil error handling improvements

Handling nil errors on addDefaultLDAPResponseDescriptions (#219)

* Handling nil errors on addDefaultLDAPResponseDescriptions

* Moving changes to v3

* Reconvering the changes from root directory

Add Digest MD5 Bind support

Add DIGEST-MD5 bind support (#230)

* Add DIGEST-MD5 bind support

* Remove new 1.13 %w verb

* Fix vet and add docs

* Fix go mod

* Fix go mod

* Update go.mod

Performance improvement and better documentation of examples

fix: memory allocations (#252)

* fix: memory allocations

* fix: replace strings.Builder to bytes.Buffer for old goland versions

* fix: rename escapedStringToEncodedBytes to decodeEscapedSymbols

* feat: remove one allocation

* fix (v3): memory allocations

Add DialURL and ModifyRequest.Increment

add modify-increment extension support detailed in RFC4525 (#227)

* add modify-increment extension support detailed in RFC4525

This extension is detailed in [RFC4525](https://tools.ietf.org/html/rfc4525).
To summarize, it allows for auto-incrementing a specific attribute by a
specific amount. It can be used in conjunction with pre-read, post-read,
and assert control types

* update v3

Use v3 suffix in module name

Update go.mod (#241)

I believe that this (plus a new tag) is what is necessary for native go modules support -- right now go modules complains that the version tag is >= v2 but the module doesn't claim to be a version >= v2.

Fix go.mod

Rename asn1 ber dependency. (#243)

* Rename asn1.ber dependency.

* go mod tidy

* Update travic CI to new asn1 ber

* Update travis go_import_path

Updated go.mod to use github.com instead of gopkg.in

v3.1.1

Use github for module name. (#239)

Add SASL/EXTERNAL mechanism for authentication

Also some internal refactoring to make the code flow more consistent and predictable.

Minor error handling fix

Also some internal Go 1.12 build updates.