* Add support for writing tEXt chunks
* Add support for reading zTXt chunks
* Add check, if keyword is valid
* Add support for reading iTXt chunks
* Add support for writing iTXt chunks
* Remove Test Decode_TextEncodingSetToUnicode_TextIsReadWithCorrectEncoding: Assertion is wrong, the correct keyword name is "Software"
* Add support for writing zTXt chunk
* Add an encoder Option to enable compression when the string is larger than a given threshold
* Moved uncompressing text into separate method
* Remove textEncoding option from png decoder options: the encoding is determined by the specification: https://www.w3.org/TR/PNG/#11zTXt
* Removed invalid compressed zTXt chunk from test image
* Revert accidentally committed changes to Sandbox Program.cs
* Review adjustments
* Using 1024 bytes as a limit when to compress text as recommended by the spec
* Fix inconsistent line endings
* Trim leading and trailing whitespace on png keywords
* Move some metadata related tests into GifMetaDataTests.cs
* Add test case for gif with large text
* Gif text metadata is now a list of strings
* Encoder writes each comment as a separate block
* Adjustment of the Tests to the recent changes
* Move comments to GifMetadata
* Move Png TextData to format PngMetaData
James Jackson-South
Brian Popow