Improvements to the execution time feature

- Running time macros have been made more portable, previously it was not
  possible to override all macros
- Running time macros will be executed by default test runner, and auto test
  runners
- Adds a default execution time implementation for unix. (Previous default
  implementation only worked on Windows)
- For embedded platforms there is a simple method of getting a default
  implementation by defining a single macro UNITY_CLOCK_MS()
- Removed need for UNITY_EXEC_TIME_RESET. This was not being used for the default
  implementations, if anything ever did need reset-like functionality it could
  simply be wrapped up with the start or stop macros for that platform

auto/generate_test_runner.rb

@@ -299,6 +299,7 @@ class UnityTestRunnerGenerator
     output.puts('  Unity.CurrentTestLineNumber = TestLineNum; \\')
     output.puts('  if (UnityTestMatches()) { \\') if @options[:cmdline_args]
     output.puts('  Unity.NumberOfTests++; \\')
+    output.puts('  UNITY_EXEC_TIME_START(); \\')
     output.puts('  CMock_Init(); \\') unless used_mocks.empty?
     output.puts('  UNITY_CLR_DETAILS(); \\') unless used_mocks.empty?
     output.puts('  if (TEST_PROTECT()) \\')
@@ -315,6 +316,7 @@ class UnityTestRunnerGenerator
     output.puts('    CMock_Verify(); \\') unless used_mocks.empty?
     output.puts('  } \\')
     output.puts('  CMock_Destroy(); \\') unless used_mocks.empty?
+    output.puts('  UNITY_EXEC_TIME_STOP(); \\')
     output.puts('  UnityConcludeTest(); \\')
     output.puts('  } \\') if @options[:cmdline_args]
     output.puts("}\n")