Sensor parser

[cboettig]

• Implements a separate method for neon_read() for sensor data that adds new columns documented in #8
• neon_index() returns timestamp as POSIXct and neon_index() and neon_read() can filter on timestamp. This provides a mechanism for overriding neon_read() default stacking mechanism of using the most recent version of a file.

[cboettig]

Also:

• removes neon_store(), closes #10
• exports neon_export, neon_import, closes #9