Add LICENSE file

[silas]

I'm packaging asyncmongo for Fedora and the reviewer asked me to ping upstream and request a LICENSE be included.

Thanks!

https://bugzilla.redhat.com/show_bug.cgi?id=654544

Non-mandatory items:
* no license file shipped in source but since all source files header mention
the actual license (vanilla ASL 2.0) it should not block the acceptation of the
package. Anyway, i recommend packager asking upstream to add one and fix the
distutils PKG-INFO file (the license field says UNKNOWN).