Optimize inbound finder parsing

`inboundfinder.py`'s `getInboundRevisions` function had a number of optimization opportunities:

1. The *.txt regex was also matching *.txt.gz log files, so the script would download ~20 ~1MB *.txt.gz log file *per revision* then attempt to search the .gz file for "/rev/". [This directory](http://inbound-archive.pub.build.mozilla.org/pub/mozilla.org/firefox/tinderbox-builds/mozilla-inbound-macosx64/1388768786/) is a good example of many *.txt.gz log files.

2. Once the *.txt regex found the correct revision txt file, the function did not break from the loop early, so it would continue download all the mismatched *.txt.gz files from item 1 above.

3. Replacing urllib2 with the [requests library](http://docs.python-requests.org/en/latest/), which will automatically opt-into and handle "Content-Encoding: gzip". This is a small help for downloading the ftp directory pages, but not help for download the already-compressed Firefox installers.

4. I didn't implement it, but a more difficult `inboundfinder.py` optimization would be to lazily parse the revision directories because most of them will not be needed. The build timestamp directories are in chronological order and, assuming the given --good-rev will precede the --bad-rev, `inboundfinder.py` could linear search the timestamp directories from the beginning for the good revision's timestamp directory and then linear search backwards from the end for the bad revision's timestamp directory. Then the `Bisector` could binary search the timestamp directories, parsing just the directories' revision txt files needed.