Implemented basic callback based SSL certificate validation.

Still missing host name validation and this commit contains some log output that will be removed prior to release.

The SSLPeerValidationMode enum is also still subject to change and SSLPeerValidationData isn't very useful, yet.
Implement basic SSL host name verification. See #469.
add Travis CI configuration for unittests
changed argument type from string[string] to FormFields
Delete pure modificator from functions toAddressString and toString, because they are impure
added convenience method for appending to a file (just like nodejs's fs.appendFile)
Merge pull request #488 from MartinNowak/fixCompile

fix compilation
Merge pull request #487 from Zaramzan/master

Fixed compilation on dmd 2.063.2
Merge pull request #486 from MartinNowak/travisCI

add Travis CI configuration for unittests
make it grammatically sound
Merge pull request #489 from Extrawurst/master

added convenience method for appending to a file...
Add travis-ci build status image.
Fix compile error in unit test.
Fix Libevent2UDPConnection.recv to properly call acquire/release to get read event notifications.
Start execution of tasks started outside of an event loop only when the event loop has started to run.
Start worker threads lazily before the first worker task is started.
Rename st_workerTask(Mutex/Signal) to st_thread(Mutex/Signal) to account for their broader scope.
Replace string parameter by const(char)[]
Merge pull request #494 from schuetzm/http_log_const_char

Replace `string` parameter by `const(char)[]` in `HTTPLogger.writeLine()`
Avoid use of BinaryHeap!Array on DMD < 2.065 trying to work around compile/link errors.

Instead, a slower RedBlackTree is used on older compiler versions now.