Implement ADC subsystem in digital pins.

Basic usage:

```c++
 PortB1 pin;
 pin.analog.read();
```

Note that AdcX (where X is number) is alias for related pin.
Add example of adc usage
Merge adc subsystem.

Currently only teensy2 is implemented. (atmega32u4 and atmega16u4)
On other systems warning will be shown for missing definitions or
about AnalogDigitalConverter class incompatibility.

Above means that nothing is broken, but teensy2 has adc system.
Makefile: Move main file quessing to variable SRC_MAIN

In project Makefile you can write

```Makefile
SRC = $(SRC_MAIN) other.c more.cpp
```

instead of explicitely writing

```Makefile
SRC = project_name.cpp other.c more.cpp
```

IF no SRC is defined it will be SRC_MAIN
Makefile: Separate LDLIBFLAGS from LDFLAGS to add them after object files