Added implementation of interface 'BoardInterface'. Method 'getBoard()' of this interface returns
string representation of two-dimensional array of chars where cells can have below values:
 1) 'T' - tank area
 2) 'B' - bullet area
 3) '.' - empty area

Example for board of size 6x6 and two tanks of size 3x3 located at (1,1) and (4,3) and one bullet
at (2,4):

[[T, T, T, ., ., .], [T, T, T, ., ., .], [T, T, T, ., ., .], [., ., ., T, T, T], [., ., B, T, T, T], [., ., ., T, T, T]]