Use g_variant_equal() instead of g_variant_compare() to compare enums.

g_variant_compare() doesn't support tuples, and we're only interrested
in equality anyway.
Added property binding for SR_CONF_VOLTAGE_THRESHOLD.
Added a comment to CMakeLists.txt
Used cotire to enhance build performance

Cotire (compile time reducer) is a CMake module that speeds up the
build process of CMake based build systems by fully automating
techniques as precompiled header usage and single compilation unit
builds for C and C++.

Cotire is released under the MIT license.
Added .gitignore rules for cotire
Refactored Signal into Trace

This will be used for non-signal traces such as the decode trace.
Made SigSession::update_signals take sdi as a parameter
Initialise Trace widgets when the trace is received by the View
Enumerate probes when session is loaded

Fixes bug #126
Read out the sample rate when loading files

Fixes bug #123
Bump cmake requirement to 2.8.6 (needed for cotire).
Make sigrokdecode a non-optional dependancy
Added decoders list
Added empty decoder dialog
Added decoder dialog heading
Added probes to decoder dialog
Added skeleton pv::view::DecodeSignal
Add decoder after decoder dialog
Added note about required probes
Added pv::data::Decoder container object