Updated example to compile with ESP platforms

additional small enhancements in library.properties, keywords.txt and README.md

Fix int32 vs uint32 comparisons

Merge pull request #13 from angrest/12-compiler-warning

#12 compiler warning (completes fix for #11 )

Bug fix for negative temperatures

Resolves Issue 11

ready for submission to library manager index

update to library.properties

optimized internal calculation logic

• when reading weather data without starting a new measurement first, now internally cached values are returned instead of recalculating the values from the unchanged sensor data again.
• a type error was fixed
• the directory for the ESP example contained a second folder level. This is fixed.

Changed return value of begin() for simplicity

Previously, begin() would return the sensor ID leaving the user with the problem to decide if this means success or not. Now, begin() returns true if the sensor ID is 0x58 or 0x60 (meaning a BMP280 or BME280 sensor) and false otherwise.
Examples and README were updated accordingly.
Additionally, there is now an example for ESP using nonstandard ports.