Feature/writing floats better

[fedjabosnic]

• Added writing longs backwards
  • Added FIXEncoding.WriteLongBackwards extension method
  • Added unit tests for WriteIntBackwards and WriteLongBackwards
  • Improved unit tests for WriteLong
• Fixed WriteFloat implementation
  • Fixed a bug where leading zeros on the fractional part were not correctly written
    • A number like 123.000123 would be written as 123.123
  • WriteFloat now takes an extra parameter for the number of decimals
  • FIXMessageWriter forces 6dp for all floats written (same behaviour as before)
• Updated benchmark results for affected code
  • No significant slowdown due to changes