Issue #77 Allow end-user configuration of tags path relative to project root

[mshenfield]

Allow user to specify the directory, relative to the root of each project
open within Atom, where symbols-view looks for the tags file, using
the tagsDirectory config variable. This closes #77 and is a step towards
#127.

This only effects toggle-project-symbols. The impetus behind this commit
is to allow popular confirations, such as storing tags files in the .git
directory, to work. The change required additional information on each
tag object, mainly specifiying the path of the project the tag was generated
for. This was to a) easily display the project directory when multiple projects
were open and b) to programatically resolve the full path of the symbol's file
when generating symbols relative to root.

This also required adding a configuration variable tagsDirectory, which should
be a directory relative to the project root that contains the tags file.

Also refactored the reloadTags method inside of watchTagsFiles into a separate
instance method, with a different name (triggerReloadTags) so that it didn't
shadow the instance attribute reloadTags, and could be used in the configuration
watcher for the new tagsDirectory configuration variable.

Added an appropriate test to run toggle-project-symbols with a relative
tagsDirectory config set.

[mshenfield]

I feel like there is an implied AtomTag class floating around, but would like to refactor in a separate PR. We have node-ctags Tag objects returned in TagReader and TagGenerator, and there is some Atom specific data and logic scattered throughout, including projectPath added in this PR, as well as the workspace appropriate naming in viewForItem.