Many small improvements (see full commit message).

* Better preamble detection to skip most of the messages we'll likely
  not be able to decode.

* A Phase correction algorithm that improves the recognition compared
  to the previous algorithm used.

* Javascript output in debug mode, and a debug.html file that can be
  used in order to see graphically undecoded samples.

* Ability to detect cross-read messages, that are, messages that happen
  to start and end across two different reads from the device or file.

* A few bugx fixed.

* README improved.
Properly initialize ICAO cached addresses hash table.
Phase correction fixed, code reshaped to apply more corrections.

The phase correction was applied only to a subset of bits! Because of an
offset error.

The detection code layout was modified a bit to make it simpler to
implement more corrections in the future. However only phase correction
is performed currently. Slope correction, or to compensate for the cycloid
effect are two possible improvements.
Strong preamble detection even with --aggressive.

To try to decode messages with a fundamentally flawed preamble is mostly
a useless waste of CPU time.

The new aggressive mode still detects a sensible percentage of
additional messages because of the error tolerance and two-bits fixes
but does not waste your CPU time.
Remove stale aircrafts when --interactive is not enabled.
Improved Google map interace: select planes and see details.
A few bugs fixed in debug.html.
CPR decoding: southern hemisphere coords are now handled correctly.
gmap.html: no rotation jumps when selecting / deselecting planes.
Fake version info removed from gmap.html.
gmap.html: update plane flight field when new data is received.
Two typos fixed in gmap.html.
Fix longitude error for southern hemisphere.

The problem was the cprModFunction() not calculating the absolute value
of the latitude, so the table only worked for the northen hemisphere.

Thanks to @pwarren for finding the bug.

This fixes issue #13.
Fix incorrect extraction of vert_rate_sign.

See issue #26. Thanks to @vk1et for reporting and fixing.
Fixed typo in --help output for --enable-agc.

Thanks to Sanjeev Gupta for reporting.