[FEATURE] Make standalone and add optional backends

This change makes the PhpProfiler independent of Plumber, so it can be
used with having to install Plumber as well.

This is useful if you want to separate the UI for viewing profiling
results from the actual data collection.

In addition this change makes it possible to store collected profiling
data into XHGui and XHProf.io.

Basic documentation has been added.
Merge remote-tracking branch 'sandstorm/master' into standalone-addedbackends
[FEATURE] Add profiling for Neos
[TASK] Convert Index.rst to README.md
[FEATURE] Allow profiling via "Profile" annotation

Using the "Profile" annotation will track the time spent in the
annotated method using a separate timer.
[TASK] Make core classes independent of TYPO3 Flow again

This change makes the core classes (Profiler, ProfilingRun) usable
without Flow, so the package can be used with the TYPO3 CMS Plumber
extension.
[TASK] Clean up save() method in ProfilingRun
[TASK] Update documentation and composer manifest
[FEATURE] Profiling probability setting

Using the PHPPROFILER_SAMPLINGRATE environment variable the probability
with which a run will be profiled can be set.
Merge pull request #4 from kdambekalns/standalone-addedbackends

Standalone addedbackends