Release 0.15.0

[wesm]

#231 needs testing / merge, but otherwise this should be released given the Thrift 0.10 issue

[lekv]

I tested the 0.15a1 package that @dknupp uploaded to pypi and it worked with the latest version of Impala. I only ran a handful of tests though and will run the exhaustive tests, too.

[dknupp]

When we get #231 tested/merged, let's cut 0.15a2.

[lekv]

Impala's core and exhaustive test suites complete successfully with 0.15a1.