Add documentation to 'bit_t' type class

# Version 0.0.1 [0.0.1D2 'Or Silver Boston']
# Changes in bit_type.h and bit_type.cpp
	+ Switch some overloaded operators to constant ones since they don't modify
	the value of '*this'
		- unary complement ! and ~ operators
		- binary &, |, ^, NAND and NOR operators
		- comparison == and != operators
		- relation >=, >, <=, < operators
		- stream insertion << operator
	+ Switched toBinaryString method to constant since it does not modify '*this'
	+ Switch some overloaded operators parameters to constant since they are not
	modified by these operators
		- binary &, |, ^, NAND and NOR operators
		- comparison == and != operators
		- relation >=, >, <=, < operators
		- @bit parameter of stream insertion << operator
# Changes in bit_type.h
	+ Add Doxygen(c) documentation to 'bit_t' file and class members
	+ Remove duplicate inclusion of <iostream> header
# Changes in bit_type.cpp
	+ Remove unneccesary commentary lines
	+ Add Doxygen(c) file documentation
	+ Modify integer and 'size_t' 'bit_t' constructors to match C++ style
	+ Rewrite integer and 'size_t' casts since previous versions of these methods
	casted integer and 'size_t' types to 'bit_t' and not the other way around
# To do
	+ Take a look at std::rotate mechanisms, maybe replace custom rotate functions
	+ Think about relation operators overloading, or does that depend on the
	endianness
	+ Add proper documentation to newly and previously committed code
	+ Double check 'bit_t' documentation