utf8_substr fix

Should fix upgrade issues caused by PHP consider 0 equal to null
Merge pull request #1359 from opencarthelp/v1.5.6.x

utf8_substr fix
Added change log note for @opencarthelp merge. Ref: #1359

Changed version id to 1.5.6.4_rc until main release.
Updated SQL query in getTotalEmailsByProductsOrdered and getEmailsByProductsOrdered to use the limits in the correct method
Updated SQL mode in install SQL file and MySQLi adapter to accommodate servers that have strict mode enabled by default. Ref: #1347

Removed Amazon payments xls file from system folder as it can be downloaded online and has no place in the code framework.
Improved currency update method to include curl timeouts, to handle occasional timeout issues. Closes #1230
Updated change log to include info and links for 1.5.6.4 release.