Wrap `pip search`, store STDERR in build.err, rename package to avoid clash

+ Adds `pip search` functionality via `VirtualEnviornoment.search(term)` and `VirtualEnvironment.search_names(term)`
+ Move from `subprocess.check_output()` to `subprocess.Popen()` to capture `STDERR` output
+ Store `STDERR` in `build.err`
+ Rename the module to `virtualenvapi` to prevent the package stopping `virtualenv` from working (`virtualenv` is implemented in `virtualenv.py`, which was being shadowed by the `virtualenv` module installed by PyPi)
+ Throw more descriptive errors
+ Bump version number due to API breaking change (module name change)