Detect attempts to modify read-only view

Adds explicit checks for read-only snapshot before attempting to perform
a modification operation. As per ConcurrentMap/Iterator interface
specification, throw an UnsupportedOperationException if such an attempt
is made.

Signed-off-by: Robert Varga <robert.varga@pantheon.sk>
Merge pull request #13 from rovarga/check-readwrite

Detect attempts to modify read-only view