v0.5.1 - HTTP/2 bug fixes

This is a minor bug fix release that addresses issues identified in the initial HTTP/2 release (v0.5). Specifically:

• Removed a duplicate and potentially-conflicting dependency (#216)
• Correctly handle null passwords when loading .p12 files without password protection (#217)
• Increased default maximum payload size from 2kB to 4kB (#220)
• Regrettably, dropped our claim that Pushy works under Java 6, which turns out to simply not be true (#224)

For a complete list of changes, please see the v0.5.1 milestone.