Adds ESP.getCoreVersion()

• for releases, returns a string with release name (e.g. "2_3_0_RC1")
• for git version, returns a short commit hash (e.g. "dd81336b")
• git version workflow works only on Mac and Linux (needs bash and git)
• if the core doesn't contain .git repository (i.e. was obtained by downloading a .zip file), returns "ffffffff"
• on Windows returns "00000000" instead of git version

Adds ARDUINO_ESP8266_RELEASE_XYZ define which can be used in #ifdef... statements (i.e. ARDUINO_ESP8266_RELEASE_2_3_0_RC1)

Core git version is stored at known offset in binary, so eboot will also report git version during startup (next to the last line):

ets Jan  8 2013,rst cause:2, boot mode:(3,6)

load 0x4010f000, len 1384, room 16 
tail 8
chksum 0x2d
csum 0x2d
vdd81336b
~ld

Same caveats apply for this, i.e. it prints "v00000000" on Windows and prints "vffffffff" if there is no .git folder in the core directory.

Relies on platform.txt so doesn't work with Platoform.IO. May or may not work with Eclipse Arduino plugin (not tested). Should work work makeEspArduino (not tested).